The present invention relates to a two-component developer comprising a toner and a carrier for developing electrostatic images for use in electrophotography, electrostatic recording and electrostatic printing, and an image forming method using the two-component developer.
In accordance with the recent spread of computer appliances for personal users, full-color image communication as an image data communication scheme is also being widely used. Under such circumstances, conversion into full-color models is rapidly promoted principally in the marketing of lower-grade machines of printers and copying machines as data outputting means.
Various techniques employing such full-color outputting machines are known, inclusive of electrophotography, thermal transfer, ink ribbon, and ink jetting. In electrophotography, generally an electrostatic latent image is formed on a photosensitive member using a photoconductive substance and the latent image is then developed with a toner to form a toner image. The thus-formed toner image is, after being transferred onto a transfer(-receiving) material such as paper as desired, fixed by using fixing means as by application of heat, pressure, heat and pressure, or solvent vapor, to form a color image.
Full-color image formation according to full-color electrophotography is generally effected by color reproduction with color toners of three primary colors of yellow, magenta and cyan or four color toners further including a black toner. More specifically, in a full-color image forming method for example, light from an original is caused to pass through a color separation filter having a color complementary to that of a toner, and laser light based on the light having passed through the filter is caused to illuminate a photoconductor layer to form an electrostatic latent image for, e.g., a magenta image. The latent image is then developed with a magenta toner and the resultant magenta toner image is transferred onto a support material. The above-mentioned steps are repeated also by using a cyan toner, a yellow toner and black toner while effecting registration to form superposed color toner images, which are usually transferred onto a transfer-receiving material, such as paper, and then fixed to provide a final full-color image, e.g., in a hot-pressure fixation step.
When a toner is blended with a carrier to provide a two-component developer, the toner is generally charged to a polarity and in a charged amount as desired by triboelectrification with the carrier and is used to develop the electrostatic image by utilizing an electrostatic attractive force. Accordingly, in order to obtain a good visible image, the toner is principally required to have a good triboelectric chargeability.
For the above problem, many studies have been made for realizing excellent triboelectric chargeability based on improvements of developer constituent materials, such as selection of carrier core material and selection of carrier coating materials, optimization of carrier coating amounts, section of charge control agents and flowability improving agents added to the toner, and an improvement of binder resin as a toner matrix material. In recent years, there has been an increasing demand for higher resolution and higher image quality of image forming apparatus, such as copying machines and printers in the market, and as a solution for complying with the demand, the use of a smaller particle size of color toner has been proposed to realize high-quality color image formation. As the toner particle size is reduced, the chargeability of the toner tends to be increased due to increase in surface area per unit weight thereof, thus being liable to cause a reduction in image density and inferior continuous image forming performance. Moreover, due to a large chargeability of the toner, a strong attachment force acts between toner particles, thus causing a lower flowability and problems in stability of toner replenishment and triboelectrification of the replenished toner.
Further, in the case of a color toner containing no dark electroconductive substance, such as magnetic material or carbon black, charge-leakage sites are liable to be insufficient, so that the toner tends to have a large chargeability. This tendency is particularly noticeable when a polyester-based binder having a high negative chargeability is used.
Particularly, the following properties (1)-(3) are strongly desired of a color toner.
(1) A fixed color toner image is placed in such a substantially completely melted state as to make individual toner particles undiscriminatable.
(2) A color toner forming an upper color toner layer has a sufficient transparency not to hinder the hue of a lower layer-forming different color toner.
(3) Respective color toners have a good balance of hue and spectral reflection characteristics and also a good saturation.
From these viewpoints, studies have been made on various binder resins, and color toners satisfying the above-mentioned performances are still desired. In the field of electrophotography today, polyester resins are frequently used as binder resins for color toners, but a color toner comprising a polyester is generally susceptible to temperature and humidity and is liable to cause an excessive charge in a low humidity environment and an insufficient charge in a high humidity environment, so that a color toner exhibiting a stable chargeability in wide variety of environments is still desired.
A toner can be charged by utilizing a triboelectric chargeability of a resin as a toner component, but the toner chargeability in this case is unstable so that the resultant image density cannot be raised quickly at the start of image formation and the resultant images are liable to be foggy. For this reason, frequently a charge control agent has been added to the toner to provide the toner with a desired triboelectric chargeability.
The charge control agents known in the art nowadays include: negatively chargeable charge control agents inclusive of metal complex salts of monoazo dyes; metal complex salts of hydroxycarboxylic acids, dicarboxylic acids and aromatic diols; and resins containing an acidic component. On the other hand, known positively chargeable charge control agents include: nigrosine dyes, azine dyes, triphenylmethane dyes and pigments, quaternary ammonium salts, and polymers having a quaternary ammonium salt as a side chain.
However, most of such known charge control agents applicable to color toners have functionally unsatisfactory points still remaining, such as difficulty in formation of uniform highlight images and a large fluctuation in image density during continuous image formation.
Other points to be further improved may include: a difficulty in obtaining a good balance between image density and fog prevention, a difficulty in obtaining a sufficient image density in a high humidity environment, a poor dispersibility in a resin, and adverse effects on storage stability, fixability and anti-offset property of the resultant toner.
As known charge control agents, metal complexes or metal salts of aromatic carboxylic acids have been proposed in Japanese Laid-Open Patent Application (JP-A) 53-127726, JP-A 57-111541, JP-A 57-124357, -JP-A 57-104940, JP-A 61-69073, JP-A 61-73963, JP-A 61-267058, JP-A 62-105156, JP-A 62-145255, JP-A 62-163061, JP-A 63-208865, JP-A 3-276166, JP-A 4-84141, and JP-A 8-160668. Charge control agents proposed in these references are generally excellent in performance of imparting triboelectric chargeability, but few of them are satisfactory in providing a stable developing performance regardless of environmental condition change, continued use and condition of use even when used in a simple developing device structure. Few of them provide a stable developing performance in a long term of continuous image formation when used in a high-speed image forming machine. Further, many of them are affected by other toner materials, thus posing a constraint on the selection of such other toner materials.
An object of the present invention is to provide a two-component developer capable of providing high-quality images in both low- and high-humidity environments and not causing image defects with lapse of time.
Another object of the present invention is to provide a two-component developer capable of providing fog-free clear images and exhibiting excellent stability in continuous image formation.
Another object of the present invention is to provide a two-component developer exhibiting excellent flowability and capable of providing toner images which faithfully reproduce latent images and excellent transferability.
Another object of the present invention is to provide a two-component developer less liable to be affected by changes in environmental conditions such as temperature and humidity and thus capable of exhibiting stable triboelectric chargeability and causing little filming or soiling on the photosensitive member.
Another object of the present invention is to provide a two-component developer exhibiting excellent fixability and capable of providing an OHP film of excellent transparency.
Another object of the present invention is to provide a two-component developer less liable to fade and exhibiting excellent light-fasteners.
A further object of the present invention is to provide an image forming method using such a two-component developer as described above.
According to the present invention, there is provided a two-component developer, comprising: a negatively chargeable toner and a resin-coated carrier; wherein
the toner comprises at least a binder resin, a colorant and an organic metal compound,
the organic metal compound is an organic zirconium compound comprising a coordination or/and a bonding of zirconium and an aromatic compound as a ligand or/and an acid source selected from the group consisting of aromatic diols, aromatic hydroxycarboxylic acids, aromatic monocarboxylic acids, and aromatic polycarboxylic acids,
the toner includes an external additive comprising hydrophobized inorganic fine powder having an average primary particle size of 0.001-0.2 xcexcm, and
the resin-coated carrier has a 50%-particle size of 20-70 xcexcm.
According to another aspect of the present invention, there is provided an image forming method, comprising:
(I) a charging step of charging an image-bearing member,
(II) a latent image-forming step of forming an electrostatic latent image on the charged image-bearing member,
(III) a developing step of developing the latent image on the image-bearing member with the negatively chargeable toner in the above-mentioned two-component developer of the present invention to form a toner image on the image-bearing member, and
(IV) a transfer step of transferring the toner image onto a transfer-receiving material via or without via an intermediate transfer member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.